If you follow the Wilton buttercream recipe (1/2 C butter, 1/2 C shortening, 4C sifted icing sugar, 1 tsp. vanilla, 2 Tbsp milk) this makes what they would call 3 cups of stiff icing. To make medium consistency, add 1 tsp. water for each cup of icing (so add 3 tsp. to 3 cups of stiff icing to make medium consistency) and for thin icing add 2 tsp water for each cup of stiff icing.